The present invention relates to recording of information and in particular to simultaneous recording of a plurality of data digits in the form of a single hologram, followed in rapid succession by similar recordings of additional holograms each of which also represents a plurality of data digits.
In the prior art, lineal arrays and page arrays of binary data were recorded on light sensitive film in the form of Fourier holograms of the arrays of data, and rotating polygonal mirrors have been used for scanning the recording film with light during recording and readout to address various hologram locations on the film. During recording, the fringe patterns of the holograms have moved somewhat because of variations in the frequency of the signal beam with respect to the frequency of the reference beam, the undesired frequency variations of the signal beam being caused by acousto-optic modulation whose primary purpose was only to amplitude-modulate the signal beam. Upon readout, the output light beam, which has been produced in the prior art by directing a laser readout beam onto one of the holograms, has exhibited an undesirable motion because of geometric effects introduced by the rotating polygonal mirror as it scans the readout light beam across the film. Small variations in the thickness of the film on which the holograms were recorded, within the width of a single hologram, have resulted in erratic variations in location of the output beam because of prismatic deflections of the readout beam as it passed through the film. Also in the prior art, non-planar acoustic waves in acousto-optic data modulators have distorted the modulated beams.